WATER. WATER'S IMPORTANCE. Solvent Most molecules dissolved in water Reactant Water's involvement in hydrolysis reactions Product Water's involvement in condensation reactions Heat transfer medium E.g. boiling, steaming, cooling. WATER'S IMPORTANCE. Texture Juiciness, mouthfeel
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UNDERSTANDING THE PHYSICAL AND CHEMICAL PROPERTIES OF WATER IS IMPORTANT IN THE STUDY OF FOOD AND PROCESSING
Because of the DIPOLE and TETRAHEDRAL structure we can get strong H-bonding
Water capable of bonding to 4 other water molecules
Unique properties of water from other hydrides
H-bond NOT a static phenomenon
Can go from ICE to GAS
Freezing induced changes in foods (examples)
Destabilization of emulsions
Flocculation of proteins
Increased lipid oxidation
Loss of water holding capacity
Example: Effect of freezing on seafoods
Association of water to hydrophilic substances
EFFECT OF SOLUTES ON WATER
1 atm (sea level)
So does it take longer or shorter to boil an egg in the Rocky Mountains? Why?
Let's go back to our egg, what would happen if you added salt?
Recommended that you add salt to water at high altitudes
Freezing point lowering
What explains all this?
P = P*/X1
P = vapor pressure of solution; P* = vapor pressure pure solvent; X1 = mole fraction of solute; x = grams solutes in solution; 55.5M = moles of water per liter
Osmotic pressure of solutions
Water surface behaves differently than bulk phase
Like an elastic film
Due to unequal inward force
Resist formation of a new surface thus forming surface tension
Ionization of water
Keq = [H3O]+ [OH]-
[H3O]+ [OH]- = Keq = Kw (Water dissociation constant)
[10-7] [10-7] = [10-14]
HOAC H+ + OAC-
pKa = -log Ka
Keq = [H]+ [OAC]-
pH = pKa + log [salt]
What do we
call this point?
What is this point and its significance to food systems?
Overall eq: H+ + HCO3-H2O + CO2
Highly perishable foods aw > 0.9
Intermediate moist foods aw = 0.6-0.9
Shelf stable foods aw < 0.6
aw = p/pO=%RH/100
Crackers that experience a
temperature rise during
At the same moisture content which
would spoil faster?
Vit C loss
aw is a major HURDLE for microorganisms but
not the only one
A) Microbial stability
B) Chemical stability
- unstable w/o water, no H-bonding
C) Enzyme stability